Beam parameter product (bpp) control by varying fiber-to-fiber angle

ABSTRACT

An apparatus includes a laser system that includes a first fiber having an output end and situated to propagate a first laser beam with a first beam parameter product (bpp) and a second fiber having an input end spliced to the output end of the first fiber at a fiber splice so as to receive the first laser beam and to form a second laser beam having a second bpp that is greater than the first bpp, wherein the output end of the first fiber and the input end of the second fiber are spliced at a tilt angle so as to increase the first bpp to the second bpp.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 62/232,379 that was filed Sep. 24, 2015 and is related to U.S. Provisional Patent Application No. 62/190,047 that was filed Jul. 8, 2015. Both of which are incorporated herein by reference in their entireties.

FIELD

The disclosure pertains to optical fiber laser systems and beam delivery systems.

BACKGROUND

In meeting customer requirements and market demand, fiber laser systems are often characterized by a set of output characteristics variable from system to system, including output power and beam quality. For example, in some laser applications, perfect, diffraction-limited (or close to perfect) beam quality is necessary while in others a reduced beam quality is sufficient (or may even be preferred) to meet process requirements. In addition, various steps of the laser process supply chain can benefit from having predictable beam quality. Also, in designing fiber laser systems, it can be desirable to have sets of components that can be common across platforms and architectures. While methods for maximizing laser beam quality have received much attention, low cost methods of manufacturing laser systems with selectable beam quality are lacking. Therefore, a need remains for solutions to overcome these drawbacks.

SUMMARY

According to an aspect of the disclosed technology, an apparatus includes a laser system that includes a first fiber having an output end and situated to propagate a first laser beam with a first beam parameter product (bpp) and a second fiber having an input end spliced to the output end of the first fiber at a fiber splice so as to receive the first laser beam and to form a second laser beam having a second bpp that is greater than the first bpp, wherein the output end of the first fiber and the input end of the second fiber are spliced at a tilt angle so as to increase the first bpp to the second bpp.

According to another aspect of the disclosed technology, a method includes selecting a beam parameter product (bpp) increase associated with laser beam propagation from a first fiber to a second fiber, selecting a tilt angle between the first fiber and the second fiber based on the selected bpp increase, and coupling the first fiber to the second fiber at the selected tilt angle.

According to another aspect of the disclosed technology, an apparatus includes a fiber fixture situated to receive and secure an output end of a first fiber in a first position and a second fiber in a second position, a fiber tip alignment mechanism situated to align the input end of the first fiber proximate the output end of the second fiber, and a splicing mechanism situated to splice the aligned input end and output end so as to form a fiber splice having a tilt angle corresponding to a selected beam parameter product (bpp) increase associated with beam propagation through the fiber splice from the first fiber to the second fiber.

According to a further aspect of the disclosed technology, a method includes selecting a beam parameter product (bpp) increase associated with a laser beam propagation from a first fiber to a second fiber, positioning at least one of the first fiber and the second fiber so that a longitudinal axis of the corresponding first fiber or second fiber is offset from a longitudinal axis of the other of the first fiber and second fiber or from a splice position between the first fiber and second fiber, wherein the offset is associated with the selected bpp increase, and splicing the first fiber and the second fiber together.

The foregoing and other objects, features, and advantages of the disclosed technology will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view schematic of a fiber splicing apparatus embodiment.

FIG. 2 is a schematic of an example laser system.

FIG. 3 is a perspective view of an example fusion splicing apparatus.

FIG. 4 is a graph of beam parameter product with respect to tilt angle.

FIG. 5 is a table of beam parameter values related to the graph in FIG. 4.

FIG. 6A illustrates an intensity profile associated with a zero angle splice.

FIG. 6B illustrates an intensity profile associated with a non-zero angle splice.

FIG. 7 is another graph of beam parameter product with respect to tilt angle.

FIG. 8 is a table of values related to the graph in FIG. 7.

FIG. 9A illustrates an intensity profile associated with a zero angle splice.

FIG. 9B illustrates an intensity profile associated with a non-zero angle splice.

FIG. 10 is a graph of bpp as a function of tilt angle.

FIG. 11 illustrates an intensity profile for a beam transmitted through a splice with a non-zero tilt angle.

FIG. 12 illustrates an intensity profile of a beam that has propagated through a zero angle splice.

FIG. 13 illustrates an intensity profile of a beam that has propagated through a non-zero angle splice.

FIG. 14 is a graph of bpp with respect to tilt angle for several splice samples.

FIGS. 15 and 16 are flowcharts of example splicing methods.

FIGS. 17A-17B are side view schematics of an example splicing apparatus.

FIG. 18 shows an end view cross-section of an example fiber fixture.

FIGS. 19-21 shows examples of a sets of fibers arranged in position to be spliced.

DETAILED DESCRIPTION

As used in this application and in the claims, the singular forms “a,” “an,” and “the” include the plural forms unless the context clearly dictates otherwise. Additionally, the term “includes” means “comprises.” Further, the term “coupled” does not exclude the presence of intermediate elements between the coupled items.

The systems, apparatus, and methods described herein should not be construed as limiting in any way. Instead, the present disclosure is directed toward all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The disclosed systems, methods, and apparatus are not limited to any specific aspect or feature or combinations thereof, nor do the disclosed systems, methods, and apparatus require that any one or more specific advantages be present or problems be solved. Any theories of operation are to facilitate explanation, but the disclosed systems, methods, and apparatus are not limited to such theories of operation.

Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed systems, methods, and apparatus can be used in conjunction with other systems, methods, and apparatus. Additionally, the description sometimes uses terms like “produce” and “provide” to describe the disclosed methods. These terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and are readily discernible by one of ordinary skill in the art.

In some examples, values, procedures, or apparatus' are referred to as “lowest”, “best”, “minimum,” or the like. It will be appreciated that such descriptions are intended to indicate that a selection among many used functional alternatives can be made, and such selections need not be better, smaller, or otherwise preferable to other selections. Examples are described with reference to directions indicated as “above,” “below,” “upper,” “lower,” and the like. These terms are used for convenient description, but do not imply any particular spatial orientation.

As used herein, optical radiation refers to electromagnetic radiation at wavelengths of between about 100 nm and 10 μm, and typically between about 500 nm and 2 μm. Examples based on available laser diode sources and optical fibers generally are associated with wavelengths of between about 800 nm and 1700 nm. In some examples, propagating optical radiation is referred to as one or more beams having diameters, asymmetric fast and slow axes, beam cross-sectional areas, and beam divergences that can depend on beam wavelength and the optical systems used for beam shaping. For convenience, optical radiation is referred to as light in some examples, and need not be at visible wavelengths.

Representative embodiments are described with reference to optical fibers, but other types of optical waveguides can be used having square, rectangular, polygonal, oval, elliptical or other cross-sections. Optical fibers are typically formed of silica (glass) that is doped (or undoped) so as to provide predetermined refractive indices or refractive index differences. In some, examples, fibers or other waveguides are made of other materials such as fluorozirconates, fluoroaluminates, fluoride or phosphate glasses, chalcogenide glasses, or crystalline materials such as sapphire, depending on wavelengths of interest. Refractive indices of silica and fluoride glasses are typically about 1.5, but refractive indices of other materials such as chalcogenides can be 3 or more. In still other examples, optical fibers can be formed in part of plastics. In typical examples, a doped waveguide core such as a fiber core provides optical gain in response to pumping, and core and claddings are approximately concentric. In other examples, one or more of the core and claddings are decentered, and in some examples, core and cladding orientation and/or displacement vary along a waveguide length.

As used herein, numerical aperture (NA) refers to a largest angle of incidence with respect to a propagation axis defined by an optical waveguide for which propagating optical radiation is substantially confined. In optical fibers, fiber cores and fiber claddings can have associated NAs, typically defined by refractive index differences between a core and cladding layer, or adjacent cladding layers, respectively. While optical radiation propagating at such NAs is generally well confined, associated electromagnetic fields such as evanescent fields typically extend into an adjacent cladding layer. In some examples, a core NA is associated with a core/inner cladding refractive index, and a cladding NA is associated with an inner cladding/outer cladding refractive index difference. For an optical fiber having a core refractive index n_(core) and a cladding index n_(clad), a fiber core NA is NA=√{square root over (n_(core) ²−n_(clad) ²)}. For an optical fiber with an inner core and an outer core adjacent the inner core, a cladding NA is NA=√{square root over (n_(inner) ²−n_(outer) ²)}, wherein n_(inner) and n_(outer) are refractive indices of the inner cladding and the outer cladding, respectively. Optical beams as discussed above can also be referred to as having a beam NA which is associated with a beam angular radius. While multi-core step index fibers are described below, gradient index designs can also be used. Some examples include fibers that support a few modes, and can be referred to as “few mode” fibers. Such fibers have a normalized frequency parameter (V-number) defined as V=2·π·a·NA/λ wherein λ is vacuum wavelength, ‘a’ is a fiber core radius, and NA is numerical aperture. For large V-number, a total number ‘t’ of modes ‘M’ supported by a fiber is approximately M=4·V²/π²+2. For single-mode fibers, V is less than about 2.405. As used herein, a few mode fiber is defined as a fiber for which a V-number is less than about 5, 10, or 20.

In some examples disclosed herein, a waveguide core such as an optical fiber core can be doped with a rare earth element such as Nd, Yb, Ho, Er, or other active dopants or combinations thereof. Such actively doped cores can provide optical gain in response to optical or other pumping. As disclosed below, waveguides having such active dopants can be used to form optical amplifiers, or, if provided with suitable optical feedback such as reflective layers, mirrors, Bragg gratings, or other feedback mechanisms, such waveguides can generate laser emissions. Optical pump radiation can be arranged to co-propagate and/or counter-propagate in the waveguide with respect to a propagation direction of an emitted laser beam or an amplified beam. In further examples, a waveguide core can be doped with one or more passive dopants, such as Ge, P, Al, Fl, and B so as to increase, decrease, or maintain a refractive index.

A laser beam beam parameter product (bpp) is generally equal to the product of the radius of the laser beam waist and the half angle of the laser beam's divergence. The ratio of a bpp of a laser beam to the bpp of a corresponding ideal Gaussian beam provides an M² beam quality value for comparing different beams. Exemplary laser beams typically contain multiple transverse optical modes. Such multimode (or few mode) beams typically have M² values greater than about 2, whereas single-mode beams typically have M² values less than about 2. In some examples, the single-mode beams and multimode beams have M² value of less than or greater than about 1.8, 1.6, 1.5, 1.4, or lower, respectively. In typical examples, a multimode beam has at least a significant portion of the power content of the multimode beam in one or more transverse optical modes higher than a fundamental LP₀₁ mode. Beam radii are often measured from a center to position where the beam has a 1/e² value of the peak intensity of the beam, though other normalizing or averaging options may be used. Divergence angles are typically determined in the far field, such as several Rayleigh lengths from a beam focus.

Fusion splicers typically include opposite fiber fixtures situated to secure opposing ends of fibers to be spliced. The opposing ends of the fibers are arranged in a center region that includes a fusion splicing mechanism, such as pair of electrodes between which the opposing fiber ends are situated for fusing and across which an arc is generated to produce the heat for the fusion splicing. Other fusion splicers can include laser sources to generate heat for fusion splicing the opposing fiber ends, or a chemical source, such as a gas flame. After the fiber fixtures secure the opposing fiber ends in the center region, a fusion cover of the fusion splicing mechanism is lowered over the secured fiber ends. A z-axis movement stage can bring one or both of the fibers in proximity to each other for fusion splicing and an x-y-z movement stage can bring one or both of the fiber tips situated in proximity so that cores, claddings, or other reference surfaces of the opposing fiber tips are aligned before heat is applied by the fusion splicer.

In FIG. 1, an apparatus 100 includes a first fiber 102 with a first fiber end 104 and a second fiber 106 with a second fiber end 108 each situated in a fusion splicer fixture 110 so that the first and second fiber ends 104, 108 are in proximity to each other. In particular, the first and second fiber ends 104, 108 are situated at a tilt angle θ instead of being parallel to each other. By situating the first and second fiber ends 104, 108 at the tilt angle θ and fusion splicing together the first and second fiber ends 104, 108, the beam parameter product of a beam propagating from the first fiber 102 to the second fiber 104 through the splice increases by an amount associated with the angle θ. In typical examples, the amount of bpp increase increases as the tilt angle increases. In representative embodiments, the bpp increase occurs as the beam passes through the splice.

The fusion splicing fixture 110 includes fiber supports 112, 114 and clamping mechanisms 116, 118 for the first and second fibers 102, 106, respectively. The clamping mechanisms 116, 118 typically come in various sizes associated with the diameter or other characteristic of the fiber to be inserted into the fusion splicer fixture 110. As shown, the first and second fibers 102, 106 have equal outer diameters of 500 μm. The second clamping mechanism 118 is sized for 500 μm fibers so that the second fiber 106 extends generally horizontally over fiber support 114. The first clamping mechanism 116 is sized for 350 μm fibers so that the first fiber 102 extends horizontally and at a tile angle due to the position of the fiber support 112 and the mismatch between the first clamping mechanism 116 and the diameter of the first fiber 102. As shown in FIG. 1, the first and second fibers 102, 106 each include respective cores 120, 122 and claddings 124, 126 with the diameter of the core 120 of the first fiber 102 being considerably smaller than the core 122 of the second fiber 106. Thus, in the example shown, the first fiber 102 is a fiber laser fiber situated to propagate a fiber laser beam ranging from about 100 W to multiple kW and the second fiber 106 is a multimode delivery fiber situated to receive the beam and deliver the beam to a target for various applications, such as cutting, welding, etc.

FIG. 3 shows an image of a fusion splicer fixture 300 with a first fiber 302 having a 510 μm outer diameter being secured in an outer fusion splicer fixture 304 having a V-groove associated with a 350 μm fiber. The first fiber 302 extends through an inner fusion splicer fixture 306 having a V-groove for receiving the fiber. The inner fusion splicer fixture 306 is generally laterally translatable so as to position the end of the first fiber 302 in alignment a second fiber end and with a pair of electrodes 308, 310 that extend perpendicular to a longitudinal direction of the first fiber 302 and are situated to provide energy for fusion splicing of the first fiber to a second fiber (not shown). Suitable fusion splicers are commercially available from Fitel and Fujikura. However, arranging fibers, such as the first fiber 302 and the second fiber, at selectable tilt angles is generally not supported as conventional splicers seek to avoid any tilt between adjacent fiber ends to be spliced. Various other methods may be used, in view of the present disclosure, to provide the tilt angle, including translation stages adjusting relative heights and angles of fiber sections to be spliced, as well as fusion splicers suitably programmed and having suitable fiber end to fiber end adjustment mechanisms. Furthermore, one or both fiber ends of the first fiber 302 and second fiber to be spliced can also be cleaved at a selected non-zero angle perpendicular to the longitudinal fiber axes of the corresponding fiber ends.

FIG. 2 shows an apparatus 200 including a laser source 202, a cladding light stripper 204, and a delivery fiber 206. The laser source 202 can include one or more gain fibers, such as a fiber oscillator or a fiber oscillator with one or more fiber amplifiers, as well as other types of laser sources that can provide an optical beam. The laser source 202 is coupled to the cladding light stripper 204 directly or with a passive fiber section 208. The cladding light stripper 204 is coupled directly or with a passive fiber section 210 to the delivery fiber 206. At one or more locations between the laser source 202 and a fiber laser beam output 212, a fiber splice optically couples adjacent fiber sections so that a beam generated by the laser source 202 propagates through the optical fiber splice and increases the beam parameter product of the generated beam by a selected, or close to a selected, amount associated with the fiber laser beam output 212. The selected increase in beam parameter product is caused by a non-zero angle between fiber-spliced adjacent fiber sections. As the beam generated by the laser source 202 propagates through the splice, the non-zero angle causes the beam to fill additional transverse modes in the downstream fiber section. Suitable locations for the beam parameter product increasing optical fiber splice include one or more of the cladding light stripper 204, delivery fiber 206, and connecting passive sections 208, 210. In some examples, the beam parameter product increasing optical fiber splice can be situated before or between fiber gain sections, such as before a fiber oscillator or before a fiber amplifier, or after a final oscillator or amplifier fiber. The increase in bpp can be associated with the improved filling of modes of a downstream gain fiber.

FIG. 4 is a chart showing simulation results predicting the increase in beam parameter product for a single-mode beam propagating through a 14 μm diameter core propagating through a fiber splice with respective fiber sections tilted at an angle during the splice. From a baseline beam parameter product of about 0.95 mm-mrad, the beam parameter product increases smoothly with increasing tilt angle. FIG. 5 is a table corresponding to the chart in FIG. 4 showing predicted increases in various beam features as tilt angle associated with the fiber splice is increased. In general, relatively small angles, ranging from about 0.5 degrees to 5 degrees achieve suitable ranges of increase in beam parameter product. FIG. 6A shows the intensity profile of a resulting output beam after a corresponding input beam has propagated through a fiber splice, where the fiber sections that were spliced were arranged end to end at a zero angle. FIG. 6B shows the intensity profile of a resulting output beam after a corresponding input beam has propagated through a fiber splice, where the fiber sections that were spliced were arranged at an angle of 1.0 degrees.

FIG. 7 is a chart showing simulation results predicting the increase in beam parameter product for a multimode beam propagating in a 40 μm diameter core and propagating through a fiber splice with respective fiber sections tilted at an angle during the splice. A baseline beam parameter product of about 2.0 mm-mrad steadily increases at the tilt angle associated with the spliced fiber ends increases. FIG. 8 is a table corresponding to the chart in FIG. 7 showing predicted increases in various beam features along perpendicular x and y axes perpendicular to the direction of propagation, including spot size Wx, Wy, beam quality Mx2, My2, beam parameter product BPPx, BPPy, BPPr (an average of x and y), as tilt angle increases. FIG. 9A shows the intensity profile of a resulting output beam after a corresponding input beam has propagated through a fiber splice, where the fiber sections that were spliced were arranged end to end at a zero angle. FIG. 9B shows the intensity profile of a resulting output beam after a corresponding input beam has propagated through a fiber splice, where the fiber sections that were spliced were arranged at an angle of 1.0 degrees.

FIG. 10 shows simulated data of beam parameter product as a function of tilt angle along with experimental results associated with fiber splices at 0.0 degrees and 0.7 degrees. FIG. 11 shows simulated and experimental results for the intensity profile of the multi-kW beams associated with the beam parameter product results in FIG. 10 for the fiber splice having 0.7 degree tilt angle. FIG. 12 shows beam output results for a 0.0 degree fiber splice tilt angle (i.e., no tilt) and FIG. 13 shows beam output results for a 0.7 degree fiber splice tilt angle. As shown in the side graphs 1302, 1304 corresponding to the intensity profiles across perpendicular axes of the beam, a flat middle portion with small center depression is established with the increased tilt angle. Thus, the mode field distribution of the beam propagating in the receiving fiber of a spliced fiber pair changes from Gaussian (or another initial distribution) to other shapes, such as flat-top, ring-shaped, or another shape, based on the increase in splice tilt angle.

FIG. 14 shows a graph 1400 depicting the relationship between bpp increase and tilt angle for several samples of splices of fibers with equal 500 μm outer diameters that were spliced with the same Fitel fusion splicer. In a first group of samples 1402, a fusion splicer clamp fixture having a v-groove designed to receive a 500 μm fiber is used for the first fiber and for the second fiber. The corresponding splice samples show a very low angle between the two fibers of about 0.2° or less, showing effectively a zero tilt angle between first and second fibers. In general, the tilt angle between the fibers was very low, typically 0.2° degrees or less, which is similar to the angle tolerance achieved in conventional splices. In a second group of samples 1404, a 500 μm fusion splicer clamp fixture having a v-groove designed to receive 500 μm fiber was used for one fiber in each splice and a 350 μm fusion splicer clamp fixture having a v-groove designed to receive a 350 μm fiber was used for the other fiber in each splice. For example, v-groove depth, width, cross-section, can be adjusted to correspond to fibers of different diameters. The corresponding results show the presence of a tilt angle, ranging from about 0.6° to 1.2°, between the longitudinal axes of the spliced fibers after fusion splicing. The increase in angle can be attributed to a longitudinal axis offset between the second fiber and the first fiber (or splicer electrodes) provided by the 350 μm fusion splicer clamp fixture. A third group of samples 1406 includes splices formed by positioning both 500 μm fibers in undersized 350 μm fusion splicer clamp fixtures. Both fibers became offset in relation to the splicer electrodes so that during alignment of the fibers and prior to the splice, the fiber tips were angled towards the electrodes. The resulting splice includes a range of tilt angles that were concentrated between about 1.5° to 2.0°.

FIG. 15 shows an example method 1500 for increasing a beam bpp by a selected amount or into a range of bpp increases. At 1502, a bpp increase amount is selected for a beam that propagates from a first fiber to a second fiber. At 1504, a tilt angle between the first fiber and the second is selected that is based on the selected bpp increase. In typical examples, tilt angles can correspond to an angle between longitudinal axes or end faces of the first and second fibers. At 1506, the end faces of the first and second fibers are coupled, for example, with a fusion splice, so as to form the tilt angle selected at 1504 and to produce a bpp increase for the beam that corresponds to the bpp increase selected at 1502. Tilt angles are typically selected in the range of 0.2° to 5°, and more typically from about 0.5° to about 1.5°. In FIG. 16, a method 1600 is shown that includes, at 1602, selected a bpp increase that is associated with a laser beam propagating from a first fiber to a second fiber. At 1604, one of the first and second fibers is positioned so that a longitudinal axis of the respective fiber is provided with an offset from one or both of a longitudinal axis of the other fiber and a splice position where the ends of the first and second fibers are to be spliced. At 1606, the first and second fibers are spliced together. In representative examples, the offset provides a tilt angle between the longitudinal axes of the first and second fibers during the splicing at 1606.

FIGS. 17A-17B show an example fusion splicing apparatus 1700 that can produce fiber splices with selectable bpp increases. The fusion splicing apparatus 1700 generally includes an outer fiber fixture portion 1702 having a fiber receiving portion 1704 situated to receive a first fiber 1706 with a predetermined fiber geometry and an outer fiber fixture portion 1708 having a fiber receiving portion 1710 situated to receive a second fiber 1712 with a predetermined fiber geometry. For example, the predetermined fiber geometry associated with outer fiber fixture portions 1702, 1708 can be a fiber cross-sectional area or outer diameter. As shown, the cross-sectional area of the first fiber 1706 is the same as that of the second fiber 1712. Typical fiber cross-sectional areas can be associated with cladding diameters of 60 μm, 80 μm, 100 μm, 200 μm, 350 μm, 500 μm, 660 μm, or smaller or larger or having intermediate values in some examples. Fiber cross-sectional areas can also be defined with respect to a core, inner cladding, outer cladding, buffer jacketing, or other fiber layers. Fiber cores can range from single mode, to few-mode, to multimode, including 6 μm, 10 μm, 15 μm, 25 μm, 50 μm, 100 μm, and 220 μm, or larger core diameters in some examples.

The fiber receiving portion 1704 of the outer fiber fixture portion 1702 is matched to the cross-sectional area or outer diameter of the first fiber 1706 so that a longitudinal axis 1714 of the first fiber 1706 secured in the outer fiber fixture portion 1702 is aligned with a splicer energy source 1716, such as an electrode, of a fiber splicer mechanism 1718. The longitudinal axis 1714 typically corresponds to a center axis of a fiber core 1720 that can be offset with respect to a cladding but is more typically centrally positioned in the fiber cross-section. The fiber receiving portion 1710 of the outer fiber fixture portion 1708 is not matched to the outer cladding diameter of the second fiber 1712 so that a longitudinal axis 1722 of the second fiber 1712, typically centered about a fiber core 1724, becomes shifted by an offset H above the longitudinal axis 1714 and the splicer energy source 1716 with the second fiber 1712 secured in the outer fiber fixture portion 1708. In some examples, the height offset H can be produced by selecting the outer fiber fixture portion 1708 to correspond to a fiber cross-sectional area or outer diameter that is smaller than the cross-sectional area or outer diameter of the second fiber 1712. In further examples, a translation stage can be used to move the second fiber 1712 or the outer fiber fixture portion 1708 to produce the height offset H. It will be appreciated that the term is used for convenience and other designations, and the offset can be associated with a length, width, distance, dimension, shift, etc.

With additional reference to FIG. 17B, z-axis translation stages 1726, 1728 are coupled to the outer fiber fixture portions 1702, 1708 so that an end 1730 of the first fiber 1706 and an end 1732 of the second fiber 1712 can be moved proximate each other. In some examples, the ends are brought in contact, partial contact, or within 1/10, ¼, ½, or one core radius of one of the cores 1720, 1724 and generally centered in relation to the splicer energy source 1716. Proximity and alignment precision can be in the range of under a 1 μm to several μm. A fiber tip alignment mechanism can include an X, Y, or XY translatable inner fiber fixture portions 1734 a, 1734 b that are coupled to the ends 1730, 1732 and situated proximate the splicer energy source 1716 so that the fiber cores 1720, 1724 can be moved in relation to the other and brought in alignment. An alignment assistance mechanism 1736 a, 1736 b can assist with the alignment of the fiber ends 1730, 1732 by coupling light into one or both of the first fiber 1706 and the second fiber 1712 and detecting an amount of light received by the receiving fiber. The alignment assistance mechanism 1736 a, 1736 b can also include one or more cameras allowing a magnified view of the fiber ends 1730, 1732. The alignment assistance mechanism can be automated or manually controlled with a splice control 1738. The splice control 1738 and a power source 1740 are coupled to the fiber splicer mechanism 1718 so as to provide energy for the splicer energy source 1716 and to control the splice characteristics of the splice produced with the fiber ends 1730, 1732. Because the longitudinal axis 1722 is provided with the offset H, the fiber ends 1730, 1732 are aligned with the inner fiber fixture portions 1734 a, 1734 b so that the longitudinal axes 1714, 1722 form a tilt angle θ at the fiber ends 1730, 1732. The inner fiber fixture portions 1734 can include spring loaded v-grooves in which the fiber ends 1730, 1732 can be inserted and secured. With the fiber end 1732 secured as shown in FIG. 17B with the inner fiber fixture portion 1734 b urging the fiber end 1732 downward, the offset H and the distance between the inner and outer fiber fixture portions 1708, 1734 b can determine the amount of tilt in the tilt angle θ. A tilt angle, such as the tilt angle θ, can be provided for the first fiber 1706 so as to combine with the tilt angle θ for the second fiber 1712 to produce a corresponding splice tilt of 2θ.

The tilt angle θ can be selected by varying the offset H or the distance between one or both of the outer fiber fixture portions 1702, 1708, the inner fiber fixture portions 1734 a, 1734 b, and the splice location of the fiber ends 1730, 1732, or both the offset H and the distance or distances. In one example, both of the longitudinal axes 1714, 1722 are raised so as to be offset from the splice location. The fiber tip alignment mechanism then repositions the ends 1730, 1732 so that a resulting splice has an increased tilt angle θ. The bpp of a beam that propagates from the first fiber 1706 to the second fiber 1712 is increased by an amount that corresponds to the selected tilt angle θ or the selected offset H. For example, where a first bpp of a beam that propagates through an untilted splice increases to a second bpp, the first bpp of the beam after propagating through a tilted splice increases to a third bpp greater than the second bpp. Alternatively, where a first bpp of a beam that propagates through an untilted splice remains the same, after propagation through the tilted splice, the first bpp increases to a second bpp. As tilt angle θ increases, fiber splices can become more susceptible to break, burn, or other failure, though in some examples, splice samples with tilt angles of greater than 0.2° and less than 1.5° did not produce a significant percentage of failures with propagating beams with continuous powers of 800 W to 4 kW. The first fiber 1706 and the second fiber 1712 can have various core and cladding diameters. In some examples, for beams propagating from the first fiber to the second fiber, the diameter of the core 1724 is typically larger than or the same size as the core 1720. Cladding diameters can vary, and in some examples, an outer cladding diameter of the first fiber 1706 can be larger than, the same size, or smaller than the outer cladding diameter of the second fiber 1712. In some examples, the increase in bpp does not occur immediately at the splice but instead at a distance, such as several cm, downstream from the splice.

FIG. 18 shows a side cross-section of a fiber fixture 1800 that includes top and bottom portions 1802, 1804 including a v-groove 1806 in the bottom portion 1804. A fiber 1808 associated with the dimensions of the v-groove 1806 is situated in the v-groove 1806 so that a center position of a fiber core 1810 is situated at a reference plane 1812. With a different fiber 1814 that has an outer diameter that is oversized for the v-groove 1806, a fiber core 1816 of the different fiber 1814 is elevated so as to be positioned at a reference plane 1818 spaced apart from the reference plane 1812. In some examples, V-groove geometry is varied for different fixtures to correspond to fibers with different outer diameters or to correspond with positioning a reference axis of the fiber 1808 at different reference planes. In further examples, V-groove geometry can remain fixed and fixture thickness can be varied for different outer diameters or to provide a fiber with different offsets. For example, the bottom portion 1804 can have a larger thickness to correspond to a fiber with a smaller outer diameter or to reposition a fiber reference axis at a higher reference plane, such as reference plane 1818. In different examples, V-groove geometry, such as depth, width, V-slope, curved or linear sides, etc. can be varied. Thus, in some examples, custom fixtures can be fabricated to provide different offsets for fibers to be spliced. In additional embodiments, a movement stage is coupled to the fixture, such as the bottom portion 1804 of the fiber fixture 1800, to raise, lower, provide lateral movement, or fiber rotation.

FIG. 19 shows an example fiber arrangement 1900 of a first fiber 1902 and a second fiber 1904 secured in respective fiber fixtures 1906, 1908. The first fiber 1902 includes an angle-cleaved end 1910 tilted with respect to a core axis 1912 of the first fiber 1902 by an angle 90-θ. The core axis 1912 of the first fiber 1902 is aligned so as to be collinear with a core axis 1914 of the second fiber 1904. The angle-cleaved end 1910 is brought proximate a cleaved end 1916 of the second fiber 1904 that extends perpendicularly with respect to the core axis 1914. A splicing mechanism 1918, such as a pair of electrodes, is situated to fusion splice the angle-cleaved end 1910 and the cleaved end 1916 together so as to form a splice that produces a bpp increase for a beam propagating from one of the fibers to the other one of the fibers based on the tilt angle θ. In some examples, both the first fiber 1902 and the second fiber 1904 can have angle-cleaved ends, and the longitudinal axes 1912, 1914 can be rotated relative to each other to define a tilt angle at the splice that varies based on the rotation.

In FIG. 20, a first fiber 2002 is secured in a fiber fixture 2004 that is situates an angle-cleaved end 2006 of the first fiber 2002 and a core axis 2008 of the first fiber 2002 in a splice position adjacent a splicing mechanism 2010. The angle-cleaved end 2006 is at an angle θ₁ with respect to a plane perpendicular to the core axis 2008. A second fiber 2012 is secured in a corresponding fiber fixture 2014 so that a core axis 2016 of the second fiber 2012 extends at a tilt angle θ₂ with respect to the core axis 2008 and approximately intersects the core axis 2008 of the first fiber 2002 at the angled-cleaved end 2006. The tilt angle θ₂ can be the same as the angle θ₁ so that the angle-cleaved end 2006 is parallel and proximate a cleaved end 2018 of the second fiber 2012 at the splice position.

FIG. 21 shows another example splice arrangement 2100 of a first fiber 2102 and a second fiber 2104 secured in respective fixtures 2106, 2108 so that respective longitudinal axes 2110, 2112 are angled at a tilt angle θ and intersect at a splice position of a splicer mechanism energy source 2114. The first fiber 2102 and second fiber 2104 include respective cleaved ends 2116, 2118 that are perpendicular to the respective longitudinal axes 2110, 2112. A fusion splice joining the cleaved ends 2116, 2118 provides a tilt angle between the first fiber 2102 and the second fiber 2104 that produces an increase in a bpp for a beam propagating from one of the fibers to the other one of the fibers.

Having described and illustrated the principles of the disclosed technology with reference to the illustrated embodiments, it will be recognized that the illustrated embodiments can be modified in arrangement and detail without departing from such principles. For instance, elements of the illustrated embodiments, such as splice tilt angle control, tilt angle and bpp selection, etc., can be implemented in software or in hardware of a fusion splicing apparatus or a controller or computer coupled to the fusion splicing apparatus. Also, the technologies from any example can be combined with the technologies described in any one or more of the other examples. It will be appreciated that procedures and functions such as those described with reference to the illustrated examples can be implemented in a single hardware or software module, or separate modules can be provided. The particular arrangements above are provided for convenient illustration, and other arrangements can be used.

In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are only representative examples and should not be taken as limiting the scope of the disclosure. Alternatives specifically addressed in these sections are merely exemplary and do not constitute all possible alternatives to the embodiments described herein. For instance, various components of systems described herein may be combined in function and use. We therefore claim all that comes within the scope and spirit of the appended claims. 

We claim:
 1. An apparatus, comprising: a laser system including a first fiber having an output end and situated to propagate a first laser beam with a first beam parameter product (bpp) and a second fiber having an input end spliced to the output end of the first fiber at a fiber splice so as to receive the first laser beam and to form a second laser beam having a second bpp that is greater than the first bpp; wherein the fiber splice is a fusion splice having a selected tilt angle that produces a selected bpp increase portion of the difference between the first bpp and the second bpp.
 2. The apparatus of claim 1, wherein the first fiber is a first gain fiber and the second fiber is a second gain fiber.
 3. The apparatus of claim 2, wherein the second gain fiber has a core diameter that is larger than a core diameter of the first gain fiber.
 4. The apparatus of claim 1, wherein the first fiber is a final gain fiber of the laser system and the second fiber is a passive output fiber.
 5. The apparatus of claim 1, wherein the first fiber is a passive fiber and the second fiber is a system delivery fiber.
 6. The apparatus of claim 1, wherein the selected tilt angle is between 0.2 degrees and 5.0 degrees based on the selected bpp increase portion.
 7. The apparatus of claim 1, wherein the selected tilt angle corresponds to an angle between a surface of the output end of the first fiber and a surface of the input end of the second fiber.
 8. The apparatus of claim 1, wherein the selected tilt angle of the fusion splice corresponds to an angle between a longitudinal axis of the first fiber at the output end and a longitudinal axis of the second fiber at the input end.
 9. The apparatus of claim 1, wherein the selected tilt angle corresponds to a first angle between a surface of the output end of the first fiber and a plane perpendicular to a longitudinal axis of the first fiber at the output end, a second angle between a surface of the input end of the second fiber and a plane perpendicular to a longitudinal axis of the second fiber at the input end, or both the first and second angles.
 10. The apparatus of claim 1, wherein the output end of the first fiber and the input end of the second fiber include respective cleaved surfaces.
 11. The apparatus of claim 8, wherein the selected tilt angle is between 1.5 degrees and 2.0 degrees.
 12. The apparatus of claim 1, wherein the selected bpp increase portion of the increase between the first bpp and the second bpp is between 0.95 mm-mrad and 1.55 mm-mrad.
 13. The apparatus of claim 1, wherein the laser system is configured to generate the first laser beam having continuous power of between 800 W and 4 kW.
 14. The apparatus of claim 1, wherein the selected bpp portion of the increase between the first bpp to the second bpp occurs at a location downstream of the splice.
 15. The apparatus of claim 1, wherein the fiber splice is positioned between fiber gain sections.
 16. The apparatus of claim 15, wherein the fiber splice is positioned after a final oscillator.
 17. The apparatus of claim 15, wherein the fiber splice is positioned before a fiber amplifier.
 18. The apparatus of claim 1, further comprising an alignment assistance mechanism configured to assist with the alignment of the first and second fibers by coupling light into the first fiber and detecting the amount of light received by the second fiber.
 19. The apparatus of claim 18, wherein the alignment assistance mechanism further comprises one or more cameras allowing a magnified view of respective end portions of the first fiber and the second fiber.
 20. The apparatus of claim 1, wherein the bpp increase portion is zero where the selected tilt angle is 0 degrees. 